1. Field of the Invention
The present invention generally relates to a system and method for detecting the quality of an alternating phase shift mask and, more particularly, to a system and method for detecting and quantifying errors in an alternating phase shift mask.
2. Background of the Invention
Photolithography is commonly utilized in the field of semiconductor manufacturing to form features (or patterns) on a wafer substrates. In one type of photolithography, known as “binary intensity mask” (“BIM”) photolithography, a photoresist or mask formed with patterns composed of opaque areas (such as chrome) and transparent areas (such as quartz) is placed above a semiconductor layer where a pattern will be formed, such as an insulating layer or a conductive layer on a semiconductor substrate. As illustrated in FIG. 1, a mask 100, together with a substrate underlayer 10, is exposed to radiation light, which may include x-ray, ultraviolet rays or other rays. Projected light within a specific wavelength region passes through transparent areas 120 of the mask to the substrate underlayer 10, but is blocked by opaque areas of the mask 110. In this manner, the exposed areas of the underlayer 10 incident to the light can be removed in a subsequent developing process, leaving the unexposed areas to remain as features on the underlayer 10 (see FIG. 1D). Such developing steps are known as a “negative developing method”. Alternatively, patterning can be performed by a “positive developing method”, where the exposed areas of the underlayer 10 incident to the light remain, but unexposed areas are removed.
Sequentially, several deposition processes and etching processes are applied to the semiconductor substrate with the patterned underlayer to form electrodes and interconnection of semiconductor devices.
FIGS. 1B and 1C illustrate a phase energy distribution curve and a light intensity curve, respectively, at areas where light passes through transparent regions of mask 100. As shown in FIG. 1B, the phase, or energy of the light that is emitted from the transparent regions of the mask appears as approximately sinusoidal pattern. Although light is blocked at the opaque areas, the phase of the beam that is emitted at the edges of transparent areas will have a shift in phase. FIG. 1C illustrates the intensity of the emitted light beam, which roughly corresponds to the square of energy. As can be seen, the beam that is emitted from the transparent areas is dispersed at a reduced intensity at areas below the opaque areas.
As semiconductor devices become more highly integrated, it becomes increasingly important to improve photolithography techniques to form finer patterns on masks. As feature sizes and pitches shrink, however, the resolution of the projection optics begins to limit the quality of the mask pattern. Particularly, IC feature sizes have decreased into a subwavelength range, below 130 nm, which is smaller than the wavelength of light output from currently-used optical lithography equipment. Lithographers are now utilizing light with a wavelength of 193 nm for patterning critical layers of 100 nm-technology nodes.
As shown in FIGS. 1A–1D, which are representative of optics in the subwavelength range, there is significant light energy (i.e., intensity) even below the opaque areas (see FIG. 1C) because the transparent areas and the opaque areas are extremely close to each other. This “unwanted” energy results in light diffraction that significantly affects the quality of the mask profile, which ideally should be vertical, especially at the boundaries between the opaque areas 110 and the transparent quartz areas 120. When light diffraction occurs, the regions on the substrate 10 that should be dark receive diffracted light, thereby causing those regions to be blurred and not distinguishable. As shown in FIG. 1D, after mask development, the boundaries 12 of the pattern 11 are not vertical to the surface of the underlayer 10 because of the above-mentioned light diffraction effects. As a result, the quality and yield of semiconductor devices that are manufactured by this mask are poor.
With the increasing demand for manufacture of semiconductor devices in a subwavelength region, a number of “subwavelength” technologies have been introduced in the industry. Among them, phase-shift masks (“PSM”), which enable the clear regions of a mask to transmit light with a prescribed phase shift, have been introduced to control the printability of the mask.
Phase shift lithography provides a method for reducing the effects of the above-described light diffraction. One type of PSM is called an alternating PSM (AltPSM), which is shown in FIGS. 2A–2D. As shown in FIG. 2A, an AltPSM 200 is manufactured by depositing an opaque material (such as chrome) on a transparent substrate, such as quartz 240, to form opaque chrome areas 210 on the quartz that are arranged alternatively with the transparent quartz areas 220 and 230. Then, alternating quartz areas (i.e., areas 230) are etched into the mask to form 180-degree-phase-shift quartz areas. Adjacent transparent quartz areas on the mask, which are separated by the opaque areas 210, are created such that the phase of the light incident upon one of the transparent areas (such as area 230) is shifted, or delayed by 180-degrees from that of light incident upon the adjacent transparent area 220.
FIG. 2B illustrates that the light emitted from adjacent transparent quartz areas is characterized by a 180-degree phase shifting. By employing two adjacent transparent areas having respective phase shifts of 0 and 180-degrees, the light diffracted into the nominally dark regions between these adjacent transparent areas interfere destructively (to cancel out each other), such that the dark regions will remain dark. FIG. 2C is a transmission profile of the AltPSM, showing that the AltPSM will yield frequency-doubled patterns on the wafer substrate 10. FIG. 2D shows that the intensity of the energy decreases to 0 when the light passes through transitions of adjacent transparent quartz areas, thus resulting in sharp profiles on the underlayer 10. As shown in FIG. 2E, the underlayer 10 has vertical profiles 15, with a sharp and clear image contrast. The clear image contrast leads to a better resolution and better depth of focus.
As the lithographic k1 factor is reduced, the advantages associated with AltPSM increase the chances of obtaining a clear image contrast. In addition to frequency doubling, AltPSM also provides an added benefit of an improved process window and a reduced sensitivity to mask errors.
Although utilizing AltPSM is a powerful solution for improving the photolithographic technique in today's subwavelength industry, it is considered to be more demanding and expensive in comparison with use of a BIM. Particularly, an AltPSM must be evaluated for light intensity imbalance between shifted and unshifted space areas, and phase defect controllability. To maximize the benefit of using AltPSM, it is necessary that the unshifted areas and the 180-degree phase shift areas are perfectly balanced, both in transmission and in phase.
FIGS. 3A–3D are graphs that illustrate the impact of various errors by the diffraction pattern that can occur using an AltPSM. In each graph, at the boundaries between 0-degree and 180-degree phase shift areas, the numerical aperture (“NA”) 0. These boundaries between the 0-degree and 180-degree phase shift are the locations of the zeroth order of the intensity of the light passing through the mask. These figures illustrate whether there are errors occurring at the zero point, i.e., the boundary between the 0-degree phase shift and the 180-degree phase shift, because it is where the phase error and the transmission error can occur. In other words, what determines errors can be detected by light diffraction at the zero point, i.e., at NA=0.
FIG. 3A shows an ideal diffraction pattern, where no error occurs at the zero point. In this figure, the light intensity at NA=0 is 0, indicating that light passing through the boundaries between the 0-degree phase shift and 180-degree phase shift areas are perfectly canceled out. Therefore, when applying this mask in the lithography process, the areas of the underlayer which correspond to the boundaries will leave sharp lines.
There are at least three types of errors that may influence the quality of the AltPSM. Generally, a phase error may exist if an incorrect material depth is obtained relative to the refractive index of the incident light (for example, if the etched transparent areas 230 in FIG. 2A are etched such that areas are too deep or too shallow). In this case, light diffraction occurs at the position NA=0 as a phase error, as shown in FIG. 3B.
A second kind of error that may occur in a PSM is known as a critical dimension (“CD”) error. CD errors may exist if the critical dimensions of the semiconductor device (for example, a gate of the semiconductor device) is not carefully controlled. FIG. 3C shows that a light diffraction also occurs at the position NA=0 when there is a CD error.
A third type of error that may occur in a PSM is a transmission error. The occurrence of a transmission error may be attributed to a phase-shifted opening (such as an etched transparent quartz area 230 of FIG. 2), which is dependent upon etch roughness as well as electromagnetic scattering phenomena from the sidewalls of the etching opening. FIG. 3D shows there is a light diffraction at the position NA=0 when the transmission error occurs.
As shown in FIGS. 3B–3D, any imbalance in the transmission, phase or CD of the mask results in the presence of a DC component in FIGS. 2B–2D. Therefore, it is important to detect the DC component of the PSM to evaluate the quality of the mask.
There are a number of techniques for measuring CD error. For example, CD error can be detected by using a scanning electron microscope (“SEM”), looking down from the top of the mask. The CD error can be detected if there are unequal lines within two regions. However, it can be difficult and cumbersome to detect phase errors and transmission errors with a SEM and the results are often inaccurate.
As described above, since a phase shift is generated by etching into the quartz on the mask, there can be a significant amount of transmission imbalance between the 180-degree areas and the 0-degree areas due to scattering etching processes, etc. Several methods have been used to compensate for this, however, due to the non-linear behavior of transmission loss error, the compensation is generally non-uniform.
To date, the predominant method for characterizing transmission error and phase error has been to use tools such as the Aerial Image Measurement System, or “AIMS,” which rapidly evaluates the exposure and depth-of-focus (“DOF”) characteristics of masks before resist experiments are undertaken.
FIG. 4 is a schematic diagram of an AIMS tool. An AIMS tool 400 basically includes an illumination device 410 for projecting an incident light with a specific wavelength region, a microscope device 420 for detecting images formed on the mask, and a platform 430 positioned between the illumination device 410 and the microscope device 420. A mask (such as an AltPSM 200) is placed on the platform 430 for detection. The components in the tool are configured such that the illumination system 410 projects light from the back of the mask 200, and the microscope system 420 receives takes an image of the mask 200 from the front of the mask 200. The image of the mask 200 is then input to a computer software 440 for an analysis of mask quality.
The illumination device 420 includes an illumination source 411 for projecting at least a deep-UV light (365-, 248-, 193-nm or other wavelengths) and a narrow-band filter 413 for establishing a center wavelength (365-, 248-, 193-nm or other wavelengths) with a bandwidth of typically <10 nm FWHM. The coherence or “σ” of the light incident upon the mask 200 is adjustably controlled by a σ-aperture slider 415 positioned at a point in the base of the microscope device 420 conjugate with an objective lens 421 of the microscope device 420. The illumination device 410 further comprises a condenser lens 417 that focuses the light onto a small (submillimeter) region of the mask 200. The platform 430 can be moved up and down for an operator to select a through-focus image data of the mask 200. The microscope device 420 includes, in addition to the objective lens 421, at least a numerical aperture (“NA”)-defining slider 423 for controlling the numerical aperture of the microscope 420 and a CCD camera for receiving the image data of the mask 200. The image data of the mask 200 is then outputted to computer software for analyzing the quality of the mask image to determine whether any mask defects exist and to measure the printability of the mask, etc.
AIMS tools are widely used for detecting the quality of photolithographic masks because the system can detect the quality of the mask before it is used in manufacturing semiconductor wafers. When the defects of a mask are detected, the mask can be discarded or repaired and then reevaluated by AIMS 400 for further detection. Alternatively, in some instances, the semiconductor manufacturing process can be adjusted to compensate for defects in the mask. In this manner, the quality of the semiconductor device can be effectively controlled and accordingly, the yield of the semiconductor device can be increased.
Although AIMS tools can effectively detect mask defects, use of such tools is very complex and expensive. Moreover, as AIMS tools are applied to detect larger areas of the mask, complicated software is required to calculate the mask patterns, thereby further increasing the cost of employing the AIMS system. However, for many masking manufacturing processes, it is not necessary to detect large areas of the mask and more particularly, to use expensive detection tools and complicated software to evaluate mask quality. A cheaper and simpler method and system for detecting the quality of the AltPSM are therefore needed.